FIELD OF THE INVENTION
The invention relates to a method for monitoring the functional capability of a tank venting system that traps fuel fumes and leads to an internal combustion engine for a motor vehicle, on the basis of a negative pressure generated in the tank venting system, including a container adsorbing fuel fumes and communicating with a fuel tank through a venting line and with an intake tube of the engine through a regeneration line; an aeration line communicating with the atmosphere and being closable by a shutoff valve for monitoring the tank venting system; a pressure sensor detecting the system pressure of the tank venting system; a tank venting valve being disposed in the regeneration line, being opened to supply the fuel fumes stored in the container and building up a negative pressure in the tank venting system; wherein the tank venting system is classified as currently nonfunctional if the pressure gradient upon buildup of the negative pressure (negative pressure buildup testing) is below a threshold or the pressure gradient upon letup of the negative pressure (negative pressure letup testing) is above a further threshold; and additionally operating variables of the vehicle including of the engine and the tank venting system are monitored, and the method is discontinued if predetermined operating variable values at which a reliable statement about the functional capability is possible, are not attained.
One such monitoring method and an apparatus therefor are known from German Published, Non-Prosecuted Application DE 41 32 055 A1, corresponding to U.S. Pat. No. 5,398,661.
That disclosure describes a tank venting system which has a tank with a tank pressure sensor, an adsorption filter which communicates with the tank through a tank connection line, and an aeration line closable by a shutoff valve, and a tank venting valve that communicates with the adsorption filter through a valve line. A tank venting system which is constructed in that way is checked for functional capability by the following method:
--monitoring of vehicle operating variables, including the engine and the tank venting system, and discontinuation of the test if predetermined operating variables at which a reliable statement on the functional capability is possible, are not attained;
--closure of the shutoff valve;
--opening of the tank venting valve;
--measurement of the negative pressure building up in the tank;
--monitoring of vehicle operating variables that first become measurable during the test procedure, including the engine and the tank venting system, and discontinuation of the negative pressure buildup test if the operating variables indicate that the measured tank pressure values do not allow a reliable statement to be made about the functional capability of the system;
--finding the system to be currently nonfunctional, and termination of the method, if the negative pressure buildup gradient is below a threshold;
--closing the tank venting valve;
--measuring the negative pressure letting up in the tank;
--monitoring vehicle operating variables that are first measurable during the test procedure, including the motor and the tank venting system, and discontinuation of the negative pressure letup testing if the operating variables indicate that the measured tank pressure values do not allow a reliable statement about the functional capability of the system;
--finding the system to be currently nonfunctional, if the negative pressure letup gradient is above a threshold, and otherwise finding the system to be currently functional; and
--opening the shutoff valve and terminating the method.